A hitherto unsolved need has appeared in the marketplace for high quality video recording tecniques employing a narrow bandwidth. At the same time, a hitherto unsolved need exists to maintain existing television recorder standards, particularly in the widely proliferated low prime cost home video cassette recorder (VCR) marketplace.
Low cost VCR's typically employ a color-under spectral format, of the type graphed in FIG. 1, a typical RF energy spectrum for NTSC color subcarrier format television picture information. At the low end of this spectrum, the chrominance information is recorded in quadrature as amplitude modulation carried by e.g. a 629 KHz color carrier. AM color sidebands extend outwardly from the color carrier approximately 500 KHz on both sides thereof.
Luminance information is recorded as frequency modulation at a higher frequency. For example, in the popular VHS home recorder format as illustrated in FIG. 1, the sync tips are at about 3.4 MHz, black level is at about 3.7 MHz, and white level is at about 4.4 MHz. A lower luminance sideband extends downwardly in frequency to about 1.2 MHz and an upper luminance sideband extends to about 5.5 MHz. The upper end tends to be reduced in amplitude in playback, as it is a function of the characteristics of the particular magnetic tape medium, the recording/playback head, and the electronics circuitry of the particular VCR machine and its design.
Tapes and machines vary widely in effective bandwidth, with the result that the poorer quality tapes and machines result in very-low-resolution playback images, while the higher quality tapes and machines employing e.g. comb filter processing techniques in lieu of traps (low pass filters) achieve a somewhat higher resolution. (The present inventor pioneered the use of comb filter techniques for chroma and luminance separation in color video recorders in his earlier U.S. Pat. No. 3,764,739, reference to which is made for further particulars.)
The spectrum available for the lowest sideband which carries luminance information is not wider than about 2.5 MHz for black level information. It may be somewhat higher for white level video information. It is extremely difficult to design a filter that works in the white levels or could vary as a function of the grey scale of the picture energy in the luminance path. No practical low cost implementations have yet been found for such a filter. As a result, the effective bandwidth in playback of a VCR of the VHS type, for example, is limited to about 200 to 250 lines, which results in a noticeably degraded picture, well under broadcast standards for picture quality.
One approach, which is always theoretically available to the VCR designer, is simply to raise the FM carrier frequency used by the recorder in order to take advantage of the latest advances in magnetic media and recording heads. This approach is extremely simple, but requires correspondingly better magnetic tape media. The big drawback to this approach is its limited compatibility with present systems following the same nominal record format, such as VHS. For example, when the carrier frequency is raised by 1.2 megahertz, the old tapes may be made to play back on the new machine, but the conventional machines will be completely unable to play tapes recorded at the higher frequency in the new format. A substantial need, therefore, is that there be compatibility between any enhanced resolution system and presently existing and in place systems and machines.
Prior efforts, such as applicant's prior "Low Level Signal Booster for Television Transition" as illustrated and described in his U.S. Pat. No. 4,262,304, have not been entirely satisfactory, although they have resulted in some apparent improvement in picture resolution in the playback mode.
It has been known for many years that when an object or field of view is periodically scanned in a series of parallel scanning lines and the light level variations are translated into electrical energy analogs, the energy is largely concentrated in a number of discrete energy groups distributed throughout the spectrum used. It is also known that there is very little useful energy lying between the groups, and that the spectral distance between the groups is related to the line scanning rate and the picture frame scanning rate. Most of the energy lies at the line scanning rate and the lower harmonics thereof, and at the frame scan rate and the lower harmonics thereof. In 1930 Gray taught in his U.S. Pat. No. 1,769,920, that the empty spaces between these energy groups were available to be occupied by additional signals, including both picture and sound.
Later, the adoption of the NTSC and PAL color television signal formats brought about the practical interleaving of the quadrature modulated color subcarrier between high frequency components (groups) of the luminance or baseband signal. This interleaving arrangement was realized by selecting as a subcarrier an odd multiple of one half of the line scanning rate, for example. Even with the color subcarrier present, the NTSC spectrum is still characterized by a lot of open space between the luminance energy groups.
Since the phase of the color subcarrier reverses with each successive scanning line and each successive frame, comb filters based on line delays and/or frame delays, and additive recombination circuits for present and delayed signals have been realized for separating chrominance and luminance components from each other.
While comb processing techniques have yielded stationary picture images having superior resolution, adaptivity techniques have been resorted to in order to eliminate the unwanted cross-color and cross-luminance artifacts which have appeared in the combed picture image as a result of the combing operation. Several successful approaches for adaptively controlling comb filter processing in NTSC (and PAL signal formats) are disclosed in the applicant's prior U.S. Pat. Nos. 4,179,705 and 4,240,105.
Another characteristic of the NTSC (and PAL) format is that in many cases excessive information is being transmitted or recorded, and there is no need for it. For example, the luminance high frequencies are being refreshed sixty times per second, when it is recognized that a refresh rate of fifteen times per second is quite satisfactory. Another example of excessive information is that the diagonal resolution of a field of view scanned in the NTSC format is higher by a factor of the square root of two (1.414) than the resolution in either the horizontal or vertical dimension. A moderate loss of diagonal resolution in NTSC is barely perceptible by the eye and is very well tolerated by the viewer.
Recently, substantial picture contrast improvements have been realized in color television picture tubes, and there has been a trend toward more use of projection television systems by the ordinary consumer. The resolution limitations inherent in broadcast quality NTSC color television signals (and PAL format signals), as well) have led to certain proposals for "enhanced resolution" and "high resolution" tevelesion systems that may or may not downwardly compatible with existing NTSC and PAL color schemes.
Several proposals have recently been made for compacting a greater number of spectral energy groups associated with enhanced or high resolution television into a given spectrum or bandwith, so that "high definition" television may be transmitted within the same spectrum presently devoted to standard definition television, such as NTSC, PAL, SECAM and the like.
One proposal is known by the acronym MUSE, for "multiple sub-Nyquist sampling encoding". One report of this technique is found in an article entitled "Development of HDTV Receiving Equipment Based on Band Compression Technique (MUSE)" by Kojima et al., appearing in IEEE Transactions on Consumer Electronics, Vol. CE-32, No. 4, November 1986, pp. 759-768. MUSE is a data compression scheme which achieves bandwidth compression by employing a sampling process by which each picture element ("pixel") is sampled once every fourth field (i.e. 1/15th of a second). Thus, four fields are required to reconstruct the entire image. This works perfectly well for stationary images. When motion is present, four times less information is available relating to this motion. However, in the MUSE proposal, a motion vector is developed. The motion is measured and the one in four sampling is carried out as the average rate of the moving object. The premise underlying this sampling approach is that the eyes and brain are relatively less sensitive to moving objects than to stationary objects. Thus, the resolution loss incident to the motion is not as objectionable to the viewer as the same loss would be for a stationary object. MUSE is one serious attempt to achieve bandwidth compression by a process somewhat analogous to spectrum folding.
Researchers led by Dr. Fukinuki at the Central Research Laboratory of Hitachi, Ltd., Tokyo, Japan, have proposed an extended definition television scheme for the NTSC format, which is an attempt to make NTSC look like high definition television. This approach is for transmission paths, such as broadcast signals, and it calls for the high frequency luminance frequencies have e.g. 4.2 MHz to be folded over and placed either under the chrominance sidebands or above the chrominance sidebands, and interleaved with the chrominance sidebands. The phase of the folded over high frequency luminance remains constant, while the phase of the chrominance sidebands reverses every other frame. Thus, a frame type of comb filter may be employed to separate the chroma from the luminance, and the folded over high frequency luminance may be reinserted at the upper end of the picture spectrum. This approach is reported in a paper by Dr. Fukinuki et al. entitled "NTSC-Full-Compatible Extended-Definition TV-Proto Model and Motion Adaptive Processing", IEEE Communications Society, Global Telecommunications Conference, Dec. 2-5, 1985, pp. 4.6.1-4.6.5.
A third high definition television proposal is set forth in an article by M.J.J.C. Annegarn, et al, entitled "HD-MAC: a step forward in the evolution of television technology", Philips Technical Review, Vol. 43, No. 8, 1987 (published in 1986), pp. 1-16. In this system each 64 microsecond line period contains in sequence, a first portion for flyback which may contain synchronizing information and sound burst packets, a second portion for color with each line carrying one and the next line carrying the other of the two color difference signals U and V in PAL, (I and Q in NTSC), and a third portion containing compressed luminance wherein the data is acquired by sampling at a sub-Nyquist rate.
While these various proposals have been made for entirely new high definition television systems and formats, none of these approaches have appreciated the need for effective comb filter preprocessing and postprocessing of the folded spectra; and, the need dhas remained unsolved heretofore for simple, yet highly effective methods and apparatus for enabling limited bandwidth video records of conventional design to record and reproduce full spectrum luminance signals.